Method of making ray sensitive targets



4 s. RAY

METHOD OF MAKING RAY SENSITIVE TARGETS Filed larch 24, 1955 Aug. 19, 1958 2 Sheets-Sheet l INVENTOR. SIDNEY ERHY Aug. 19, 1958 s, GRAY METHOD OF MAKING RAY SENSITIVE TARGETS Filed March 24, 1955 2 Sheets-Sheet 2 21!!!!III a a a o INVENTOR. SIDNEY GREY 2,848,358 METHOD OF G RAY SENSITIVE TARGETS Sidney Gray, Somerville, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application March 24, 1955, Serial No. 496,410 14 Claims. (Cl. 117-211) This invention relates to devices utilizing closely spaced conductive elements electrically insulated from each other and formed from vaporized conducting material, and more specifically to electron discharge devices employing targets having such construction and to methods of making such devices.

In various types of cathode ray tubes suitable for color television it has been proposed to provide a ray sensitive target which includes a plurality of fine closely spaced elements. It is often desirable to interconnect these elements in one relation or another, as in the case of pickup tubes, for producing simultaneous color video signals. One of such pickup tubes is capable of producing several video signals each corresponding to a color component of the object light. For example, three signals, each corresponding to a different one of the three primary colors of the object light, are taken from a signal output means of the pickup tube and applied to the utilization circuit. It has also been proposed to eifect various improvements in such tubes by providing a structure in which the corresponding elements for producing each of stantially in a single plane. In such an arrangement, the signal output means which is capacitively coupled to a varying charge pattern, or electron image, formed by the object light on a surface scanned by the electron beam, is made up of a plurality of spaced parallel conductive strips or segments.

More specifically, in one form of device the ray sensitive target includes a transparent insulating support, such as a sheet of glass. A plurality of parallel light filter strips, made of insulating material, extend across a surface of the support and include filters for the diiferent colors desired. The transparent conductive signal strips are superimposed on the filter strips and are spaced from each other. A layer of photosensitive material, which is to be scanned by an electron beam, covers the signal strips. While the precise number of filters and conductive strips is not at all critical, it will be readily appreciated that they must be extremely narrow to avoid undue enlargement of the pickup tube and at the same time form a signal image having sufiiciently high definition, elemental portions of which conform to elemental portions of the object image, of a size acceptable to the observer at the receiver. In the three color type of pickup tube, the conductive strips forming the signal plate are connected in sets to form three separate but interlaced signal output means, one for each color. The conductive strips are arranged with a strip for each color such as red, green and blue in a given sequence which is repeated throughout the target. The conductive strips for the red are connected together, as are also the strips for the green and blue.

The signal strips are preferably at least semi-transparent and are highly conductive. One material which has been found to be suitable is a thin strip of gold which is normally fabricated by evaporating material through a masking grill. In practice it has been found diflicult to form such targets which are wholly satisfactory in operation.

the color signals are subsit) One of the faults noted is color cross-talk, or color dilution, caused by high electrical leakage of signal between adjacent signal strips. High leakage between sets of signal strips is attributed primarily to a scattered film of gold which is formed between the signal strips, i. e., the areas in the shadow of the grill wires during the evaporation process. It has been found diflicult to entirely eliminate this scattering even though the masking grill wires are brought close to the surface to be coated.

An object of this invention is to provide an improved method of manufacturing closely spaced conductive elements that are insulated from each other.

Another object is to provide an improved method of and means for making devices utilizing closely spaced conductive elements insulated from each other and formed by vaporizing conductive material through a mask.

The above and other objects are achieved in accordance with this invention by either providing a method including the use of a masking grill having a surface which considerably reduces the amount of gold scattering during evaporation or by a method including preparing the surface on which the gold is deposited so that a given thickness of scattered gold will be less conducting, or both.

In the two sheets of drawings:

Figure 1 is a longitudinal sectional view of a pickup tube constructed in accordance with the invention;

Figure 2 is an enlarged fragmentary view of the target of the tube shown in Figure 1 and constructed in accordance with the invention;

Figure 3 is an enlarged fragmentary sectional view of the target shown in Figure 2;

Figure 4 is a diagrammatic view of apparatus for forming the target according to the invention; and

Figure 5 is an enlarged fragmentary sectional view of the target taken during one step of the method according to the invention.

The invention will now be describd in detail as used in a photoconductive pickup tube wherein it is particularly useful. However, the invention is considered to be useful also in other types of tubes and particularly cathode ray tubes suitable for use in color television transmission.

Referring now to the drawing in detail, and in particular to Figure l, pickup tube 10 comprises an evacuated envelope 11 with an electron gun 12 mounted at one end thereof. The electrodes of the electron gun include the usual cathode 30, control electrode 31 and one or more accelerating anodes 32 which are connected to lead-ins 33 in the well known manner. An electron beam from the gun is directed upon a target 13 at the other end portion of tube 10. Means are provided for focusing the electron beam and scanning the beam over target 13 to form a raster and may include a focus coil 14 and a deflection yoke 15 as well as an alignment coil 16. An electrode 17, which is permeable to the electron beam, and which may be made of mesh material, is positioned adjacent to target 13 and in operation, together with focus coil 14, functions to insure that the beam in its final approach to the surface of target 13 is normal thereto. A final accelerating electrode 18 is in the form of a conductive coating on the interior of envelope 11. Fingers mounted on the gun 12 but insulated therefrom serve to connect the coating 18 to one of the lead-in pins.

The target 13 is conveniently supported adjacent to a transparent window 20 and terminal pins 19 sealed through the envelope are connected thereto. As shown most clearly in Figures 2 and 3, the target 13 comprises a support or backing 21 preferably of transparent glass. Conductive coating material in the form of strips, or bus bars 22, are formed in the present instance on the gun side of supportZl. As shown, two sets of bus bars 22 are provided; a set being located adjacent to opposite.

24 are not critical, the strips are extremely fine.

- inch.

ends of backing 21. Each set of bus bars 22 includes one bus bar for each of three primary color components of the light from an object for which a video signal is desired. Three separate color filters, for example red, green and blue, are formed by laying down suitable material to form filter strips 23. One type of filter which has been found suitable, for example, is a multi-layer interference type filter which can be made to have the desired pass bands for the selected primary colors. A sufiicient number of alternate layers of a high and low index of refraction material having an optical thickness at a desired wavelength are formed. The filter strips 23, in thepresent instance are of insulating material. The red filterstrip R extends over the blue and green signal conductive bus bars and serves to insulate transparent conductive signal strips 24 from those bus bars at the cross over points. Each of the filter strips overlies bus bars from which its respective signal strips is to be insulated.

Transparent conductive signal strips 24 are formed of suitable coating material, in this instance gold. While the specific dimensions of transparent conductive strips At the present time, as many as 720 such strips are formed per The signal strips 24 are spaced approximately one half .mil. (.0005 inch) apart from edge to edge and are approximately 150 angstrom units thick. Signal strips 24 are each laid down over the associated filter and are longer than the filter so as to make electrical contact with the proper bus bars 22. In the form shown in Figure 2, the sequence of red, green and blue filters, each with its associated transparent conducting signal strip, is repeated over the target with adjacent ones off- .set as shown.

Thus, target 13 has an interlaced network of the several color filters 23 and transparent conducting strips 24, with the conducting strips 24 associated with the filters of the same color each connected at one end to one common bus bar 22 and at the other end thereof to another common bus bar 22. Each set of conducting strips is connected to one of the terminal pins 19 by a suitable lead connected to both of the bus bars 22 common to that set.

Photoconductive material is laid down over the conductive strips 24 to form a photoconductive coating 26. The coating 26 is preferably of photoconductive material having a broad spectral response similar to that of the human eye, one example of which is porous antimony trisulphide, though other materials may be used.

In operation, the target 13 may be oriented as desired in the tube 10. For example, the filters 23 and signal strips 24 may be oriented parallel or perpendicular to the direction of scan. Pickup tube may be operated in several ways and either low or high velocity operation is suitable.

Insulating red and green filters of the type referred to above may be formed by evaporating successive layers of zinc selenide and magnesium fluoride onto support 21. Zinc selenide has a high index of refraction, and magnesium fluoride has a low index of refraction. For example, nine layers of Zinc selenide and magnesium fluoride formed in that sequence provide suitable red filters, and 10 layers for green.

For the red filters, the optical thickness of each of the Zinc selenide layers, except the central one, is three quarters wave length of light having a Wave length of approximately 4950 angstrom units; the central zinc selenide layer and each of the magnesium fluoride layers is one quarter wave length for light of 4950 angstrom units wavelength.

For the green filters, the optical thickness of each of 'the zinc selenide layers, except the central one, is three ers is one quarter wave length for light of 6300 angstrom units wavelength.

For forming the blue filters, zinc sulfide and magnesium fluoride may be used, and twelve layers have been found to be suitable. Here, the zinc sulfide and magnesium fluoride are preferably laid down in layers having an optical thickness of one quarter of a Wave length of light of about 5430 angstrom units wave length.

Bus bars 22 and conductive strips 24 are preferably formed of gold. Where, as in the present instance, conductive strips 24 need also to be transparent, extremely thin strips are formed. Gold is an especially desirable material for the conductive strips 24 for several reasons. In the first place it is chemically inert with respect to gases, such as oxygen, and with respect to reactions with the photoconductor or filters. Secondly, it may be readily deposited by evaporation techniques. Further, its optical absorption is low. In addition, high conductivityis obtainable.

Such a target as described above may be formed by depositing the various materials by evaporation, in proper sequence, through suitable masks to form the fine closely spaced as well as superimposed strips. The evaporation of the various materials is carried out in a suitable atmosphere. Bus bars 22, filters 23 and conductive strips 24 are laid down in a vacuum of about 1x10 mm. of mercury when the materials indicated above are used. Photoconductive material such as porous antimony trisulphide may also be evaporated in a vacuum of about 0.6 mm. of mercury to form the photoconductive layer.

As indicated previously, one of the problems encountered in making targets of the'type above described is the undesired formation of a conductive film of gold in the spaces between the conductive strips 24. This film is produced by the scattering of gold vapor particles by the masking grill wires during the evaporation process when forming the conductive strips 24. It is believed that the scattered film results from vapor particles which strike the grill wires and are deflected so that they are deposited in the spaces between the conductive strips 24.

Referring to the apparatus shown in Fig. 4, the target 13 is supported within a bell jar 31 by a holder 27 and held in close proximity to the Wires 28 of the masking grill through which gold is to be evaporated from a source 29. The bell jar 31 is connected to an exhaust apparatus (not shown) so as to produce the desired amount of vacuum for carrying out the evaporation process. The source 29 may include a crucible for holding the gold to be evaporated, and an electric heater coil may surround the crucible to heat the gold to vaporization. Or, if desired, the heater coil itself may constitute the crucible for holding the gold. As shown more clearly in Fig. 5, the grill Wires 28 should be spaced a slight distance from the surface to be coated so as not to injure the surface. Scattering is difficult to prevent even with very close spacing of the grill.

The masking grills used for strip fabrication are generally wound of Nichrome Wire, which is an alloy of percent nickel and 15 percent chromium. Nichrome is preferred for its superior mechanical properties, because the Wires have to hold under great tension to remove any kinks. However, it -has been found that with freshly cleaned Nichrome wires scattering of gold occurs to an extent which cannot be tolerated in acceptable devices.

In accordance with my invention, I solve the problem of high leakage between conductive strips formed by vaporization by either modifying the surface upon which the conductive strips are deposited or the masking structure through which the material is deposited, or both. In accordance with one phase of my invention, by precoating wires 28 with evaporated gold 30 to -a thickness of about angstroms, I have been able to reduce scattering to a considerable degree. The gold coating provided on the grill wires causes the vapor particles which strike 5 it to adhere more readily than they do when no coating is provided.

The property of the grill wire surface which characterizes its ability to cause incident evaporated material to adhere to it, rather than be deflected from it, will hereinafter he referred to as its accommodation coefiicient. Hence, a material having a high accommodation coefiicient for gold is one for which 'gold incident on it in a vapor stream has a high probability of adhering to it.

The accommodation coetficient of the grill wire surface for evaporated gold depends on the surface proper ties of the maskin wire, for example its crystal structure, and the presence of a surface layer of adsorbed gas. A precoated grill loses some of its ability to reduce scattering if it is permitted to stand exposed to room atmosphere for a considerable period. On the other hand, if such a grill is stored in vacuum between uses, it maintains its efiiciency.

, I have also obtained a high accommodation coefficient for gold by depositing evaporated zinc sulfide on wires 28 and thereby reduced gold scattering.

Gold and zinc sulfide are especially good materials for 'precoating the grill wires because, in addition to reduc ing scattering, they have other desirable properties. They :are both evaporable materials and hence can be laid down on the grill wires in films thin enough so as not to unduly increase the diameter of the grill wires. Furthermore, they will adhere to the grill wires rather than fiake off and cause damage to the target structure.

I have further reduced the leakage effects of scattered gold by a coating laid over the filter strips of the target beneath the signal strips. As shown in Fig. 3, the surface of the target 13 on which the conductive signal strips 24 are formed is treated in accordance with this invention in such a manner that a given quantity of scattered gold is made less conducting. As indicated previously, a transparent layer of insulating material 25 is deposited over the surface of the filter strips 23 prior to the deposition of the conductive strips 24. The layer 25 is a material having surface properties such that gold atoms, or aggregates of gold atoms, are highly mobile thereon. The object is to provide a surface which is not readily wetted by the gold. With layer 25, it is believed that what small amount of gold is scattered in the wire shadows will tend to ball up and form isolated globules rather than a continuous film. However, in the areas in registry with the spaces between the wires, the amount of gold deposited is so much greater than in the scattered areas that the globules run together and form continuous conductive signal strips.

A suitable material for the layer 25 is a film of aluminum oxide. Such a layer may be formed by depositing evaporated material as a film of aluminum, which is 80-85 percent transparent, over the filter structure. This corresponds to a film which is about l20 angstrom units thick, and may be laid down by evaporation in a vacuum of 1X10 mm. of mercury. Such a film may then be completely made substantially transparent by exposing it to room atmosphere for several minutes to thus oxidize the film. If the evacuation is not carried too far, there will be suflicient oxygen left to convert the aluminum into aluminum oxide without exposing to the atmosphere. In either case, the resultant aluminum oxide is more transparent to light than the former aluminum film because the film no longer has the metallic reflecting or absorption properties. It is also thin enough not to interfere with the filter action. In addition, it is electrically insulating at least parallel to its surface. More important, however, is the property of the aluminum oxide film to cause any scattered gold which forms on its surface to granulate and form a discontinuous fihn.

An alternative material for film 25 is a film, or layer, of silicon monoxide which may be deposited in a manner similar to that described for aluminum. The silicon monoxide will then oxidize to the desired properties.

Practicing the above described invention and using an percent transparent layer of evaporated aluminum, oxidized to complete transparency over filter strips of zinc sulphide and using a Nichrome masking grill which had been washed, the resistance between adjacent signal strips was increased from 1,200 ohms to over 190,000 ohms when using the aluminum oxide layer. When the gold was vaporized and deposited through a grill which was precoated with gold, the interstrip resistance over the zinc sulfide substrate without the film or layer 25 of aluminum oxide was 50,000 ohms, and with the aluminum oxide layer a minimum of several hundred megohms.

Thus, by using both of the above improvements according to my invention the leakage between the conductive strips was decreased to a negligible amount and the performance of pickup tubes for use in color television greatly improved. Either teaching alone provides a major improvement in the tubes made according to my invention.

I claim:

I. The method of making a structure including a base and a plurality of closely spaced conductive elements electrically insulated from each other on a surface of said base, said method including depositing an insulating layer on said base, said insulating layer having the characteristic of causing vaporized material from which said conductive elements are formed to form a discontinuous layer during initial stages of deposition, placing on apertured masking member in close spaced relation with said insulating layer, said masking member being coated with a material having a high accommodation coetficient for the material forming said conductive elements, and directing vaporized material in sufficient quantities to form continuous conductive elements on said insulating layer.

2. The method of making a structure including a base and a plurality of closely spaced conductive elements electrically insulated from each other on a surface of said structure, said method including depositing an insulating layer on said base, said insulating layer having the characteristic of causing vaporized material from which said conductive elements are formed to form a discontinuous layer during initial stages of deposition, placing an apertured masking member in close spaced relation with said insulating layer, and directing vaporized material in willcient quantities to form continuous conductive elements on said insulating layer.

3. The method of making a structure including a base and a plurality of closely spaced conductive elements electrically insulated from each other on a surface of said base, said method including placing an apertured masking member in close spaced relation with said base, said masking member being coated with a material having a high accommodation coefiicient for the material forming said conductive elements, and directing vaporized material in sufiicient quantities to form continuous conductive elements on said base.

4. The method of making .a ray sensitive target for a cathode ray tube having transparent spaced conductive elements on said target and electrically insulated from each other, said method comprising depositing on a surface of said target a transparent insulating coating, said coating having the characteristic of causing vaporized gold directed thereon to form into a discontinuous layer, spacing an apertured masking element closely adjacent to said coating, said masking element having a high accommodation coefiicient for gold, and directing vaporized gold through said mask in sufilcient quantities to form spaced conductive elements on said coating and insulated from each other.

5. The method of making a ray sensitive target for a cathode ray tube having transparent spaced conductive elements on said target and electrically insulated from each other, said method comprising depositing on the sur silicon dioxide, which has face of said target a transparent insulating coating, said coating having the characteristic of causing vaporized gold directed thereon to form into a discontinuouslayer, spacing an apertured masking element closely adjacent to the exposed surface of said coating, and directing vaporized gold through said exposed mask in sufficient quantities to form spaced conductive elements on said surface and insulated from each other.

6. The method of making a ray sensitive target for a cathode ray tube having transparent spaced conductive elements on said target and electrically insulated from each other, said method comprising spacing an apertured masking element closely adjacent said surface, to said masking element having a high accommodation coefficient for gold, and directing vaporized gold through said mask 1 in suflicient quantities to form spaced conductive elements on said surface and insulated from each other.

7. A method of making a ray sensitive target provided with a transparent supporting member having transparent spaced conductive elements thereon and electrically insulated from each other comprising, depositing on a surface of said target a transparent insulating layer of a material on which vaporized gold directed thereon tends to form a discontinuous layer, spacing a grill wire structure in close proximity to said transparent insulating layer, and evaporating through said grill wire structure a quantity of gold to form on said insulating layer a plurality of conductive strips which are insulatingly spaced from each other.

8. A method of making a ray sensitive target provided with-a transparent supporting member having transparent spaced conductive elements thereon and electrically insulated from each other, said method comprising depositing on a surface of said target a transparent insulating layer of a material on which vaporized gold directed thereon tends to form a discontinuous layer, spacing a grill wire structure in close proximity to said transparent insulating'layer, said grill Wire structure having a high accommodation coefficient for evaporated gold, and evaporating through said grill wire structure a quantity of gold 'to form on said insulating layer aplurality of conductive strips which are insulatingly spaced from each other.

9. A method of making a ray sensitive target, said method comprising providing a support, depositing on a surface of said support a semi-transparent layer of aluminum, oxidizing said aluminum layer to form a transparent insulating layer of aluminum oxide, spacing a grill wire structure in close proximity to said aluminum oxide layer and evaporating through said grill wire structure a quantity of gold to plurality of conductive strips which are insulatingly spaced from each other.

form on said aluminum oxide layer a 10. A method of making a ray sensitive target, said method comprising providing a support, depositingon a surface of said support a transparent insulating layer of a material on which atoms ofgold and aggregates thereof are highly mobile, spacing a grill wire structurein close proximity to said transparent insulating layer, thesurface of said grill wires having a high accommodation coeflicicnt for gold, and evaporating through said grill wire structure a quantity of gold to form on said insulating layer a plurality of conductive strips.

11. A method of making-a ray sensitive target, .said method comprising providing a support, spacing .a grill wire structure in close proximity to said support, the surface of said grill wires having a high accommodation .coefiicient for gold, and evaporating through said grill wire structure a quantity of gold to form on said insulating layer a plurality of conductive strips. I

12. A method of making a ray sensitive target having insulated conductive strips thereon by means of a wire grill mask, said method comprising coating thegrill wires with a material having a high accommodation coefficient for gold, placing'said coated grill wires in close proximity to said target, and evaporating a quantity of gold through said coated grill mask to form on said target a. plurality of conductive strips.

13. A method of making a ray sensitive target, said method comprising providing a grill wire structure, coating said grill wires with gold, placing said coated grill wires in close proximity to a support, and evaporating through said coated grill wires a quantity of gold to form on said support a plurality of conductive strips.

14. A method of making a ray sensitive target, said method comprising providing a grill wire structure, coating said grill wires with zinc sulphide, placing said coated grill wires in close proximity to a support, and evapcrating through said coated grill wires a quantity of gold to form on said support a plurality of conductive strips.

References Cited in the file of this patent UNITED STATES PATENTS 2,408,614 Dimmick Oct. 1, 1946 2,411,715 Dimmick Nov. 26, 1946 2,446,791 Schroeder Aug. 10, 1948 2,590,557 Melsheimer Mar. 25, 1952 2,610,606 Weber et al Sept. 16, 1952 2,616,057 Coltman Oct. 28, 1952 2,633,427 Dimmick et a1. Mar. 31,1953 2,705,764 Nicoll Apr. 5, 1955 2,719,241 Coltman Sept. 27, 1955 

1. THE METHOD OF MAKING A STRUCTURE INCLUDING A BASE AND A PLURALITY OF CLOSELY SPACED CONDUCTIVE ELEMENTS ELECTRICALLY INSULATED FROM EACH OTHER ON A SURFACE OF SAID BASE, SAID METHOD INCLUDING DEPOSITING AN INSULATING LAYER ON SAID BASE, SAID INSULATING LAYER HAVING THE CHARACTERISTIC OF CAUSING VAPORIZED MATERIAL FROM WHICH SAID CONDUCTIVE ELEMENTS ARE FORMED TO FORM A DISCONTINUOUS LAYER DURING INITIAL STAGES OF DEPOSITION, PLACING ON APERATURE MASKING MEMBER IN CLOSE SPACED RELATION WITH SAID INSULTING LAYER, SAID MASKING MEMBER BEING COATED WITH A MATERIAL HAVING A HIGH ACCOMMODATION COEFFICIENT FOR THE MATERIAL FORMING SAID CONDUCTIVE ELEMENTS, AND DIRECTING VAPORIZED MATERIAL IN SUFFICIENT QUANTITIES TO FORM CONTINUOUS CONDUCTIVE ELEMENTS ON SAID INSULATING LAYER. 